Certain olefinic functions, particularly the allylsilane and vinyl fluoride residues, are to be studied as terminators of biomimetic polyene cyclizations, because basic studies indicate that these groups render the ring-closures highly regio- and stereo-selective. Cyclization substrates having an oxy group at pro-C-7 or at pro-C-11 promise to give tetracyclic products that can lead to spironolactone types or corticoids respectively in an enantioselective process.